The increasing operating speeds and computing power of microelectronic devices have recently given rise to the need for an increase in the complexity and functionality of the semiconductor structures from which these devices are fabricated. Hetero-integration of dissimilar semiconductor materials, for example, III-V materials, such as gallium arsenide, gallium nitride, indium aluminum arsenide, and/or germanium with silicon, silicon-on-insulator, or silicon-germanium substrates, is an attractive path for increasing the functionality and performance of the CMOS platform. Specifically, as geometric scaling of Si-based MOSFET technology becomes more challenging, the heterointegration of alternative area materials becomes an attractive option for increasing the innate carrier mobility of MOSFET channels. For many applications, it is desirable to incorporate alternative active-area materials having a low density of dislocation defects onto an insulator platform. As used herein, the term “alternative materials” refers to either a non-silicon semiconductor, or silicon with a different surface or rotational orientation compared to the underlying substrate. Such areas are suitable for use as active areas for MOSFETs or other electronic or opto-electronic devices.
Heterointegration of alternative materials has thus far been typically limited to the addition of SiGe alloys of small Ge content for use as source-drain contact materials or heterojunction bipolar transistor base layers. Since such layers are only slightly lattice-mismatched to Si, and since most modern Si MOSFET processes are compatible with these dilute SiGe alloys, few disruptions in the Si MOSFET integration sequence have been necessary. The drive for increased carrier mobility—and concomitant device drive current—will soon, however, necessitate the use of other, more highly lattice-mismatched materials for historically Si-based devices, requiring more disruptive changes to the traditional device integration flow.
In particular, heteroepitaxial growth can be used to fabricate many modern semiconductor devices where lattice-matched substrates are not commercially available or to potentially achieve monolithic integration with silicon microelectronics. Performance and, ultimately, the utility of devices fabricated using a combination of dissimilar semiconductor materials, however, depends on the quality of the resulting structure. Specifically, a low level of dislocation defects is important in a wide variety of semiconductor devices and processes, because dislocation defects partition an otherwise monolithic crystal structure and introduce unwanted and abrupt changes in electrical and optical properties. These, in turn, result in poor material quality and limited performance. In addition, the threading dislocation segments can degrade physical properties of the device material and can lead to a premature device failure.
As mentioned above, dislocation defects typically arise in efforts to epitaxially grow one kind of crystalline material on a substrate of a different kind of material—often referred to as a “heterostructure”—due to different crystalline lattice sizes of the two materials. This lattice mismatch between the starting substrate and subsequent layer(s) creates stress during material deposition that generates dislocation defects in the semiconductor structure. The stress field associated with misfit dislocations under certain conditions may cause formation of linear agglomerations of threading dislocations, termed a “dislocation pile-up.” This is generally defined as an area comprising at least three threading dislocations, with a threading dislocation density greater than 5×106 cm−2, and with threading dislocations substantially aligned along a slip direction such that the linear density of dislocations within the pile-up and along a slip direction is greater than 2000/cm. For example, the slip directions in SiGe materials are in-plane <110> directions. A high localized threading dislocation density present in dislocation pile-ups has a potentially devastating impact on the yield of devices formed in these regions and may render these devices unusable. Inhibiting the formation of dislocation pile-ups is, therefore, desirable.
To minimize formation of dislocations and associated performance issues, as mentioned above, many semiconductor heterostructure devices known in the art have been limited to semiconductor layers that have very closely—e.g. within 0.1%—lattice-matched crystal structures. In such devices a thin layer is epitaxially grown on a mildly lattice-mismatched substrate. As long as the thickness of the epitaxial layer is kept below a critical thickness for defect formation, the substrate acts as a template for growth of the epitaxial layer, which elastically conforms to the substrate template. While lattice-matching and near-matching eliminate dislocations in a number of structures, there are relatively few lattice-matched systems with large energy band offsets, limiting the design options for new devices.
Accordingly, there is considerable interest in heterostructure devices involving greater epitaxial layer thickness and greater lattice misfit than known approaches would allow. One known technique termed “epitaxial necking” was demonstrated in connection with fabricating a Ge-on-Si heterostructure by Langdo et al. in “High Quality Ge on Si by Epitaxial Necking,” Applied Physics Letters, Vol. 76, No. 25, Apr. 2000. This approach offers process simplicity by utilizing a combination of selective epitaxial growth and defect crystallography to force defects to the sidewall of the opening in the patterning mask, without relying on increased lateral growth rates. This approach, however, generally requires relatively thick semiconductor layers, as well as relatively small lateral dimensions of the openings in the mask in order for the dislocations to terminate at its sidewalls, resulting in defect-free regions.
Several methods to fabricate non-Si semiconductors on insulator substrates have been previously reported, whereby transfer of SiGe material onto insulator substrate was achieved through bonding and splitting induced by hydrogen implantation and annealing. Generally, in these approaches, a relatively thick SiGe layer is deposited on a silicon substrate, which includes a graded SiGe buffer layer and a relaxed SiGe layer having a constant germanium concentration. Following surface planarization, hydrogen is implanted into the SiGe layer to facilitate wafer splitting. The Si/SiGe wafer is then bonded to an oxidized silicon substrate. The SiGe-on-oxide layers are separated from the rest of the couplet by thermal annealing, wherein splitting occurs along hydrogen-implantation-induced microcracks, which parallel the bonding interface.
A technique to form a SiGe-free strained silicon-on-insulator substrates has been also reported by T. A. Langdo and others in “Preparation of Novel SiGe-Free Strained Si on Insulator Substrates,” published in 2002 IEEE International SOI Conference Proceedings (Oct. 2002). This technique is similar to approaches described above, except that a thin layer of epitaxial silicon is deposited on the SiGe layer before wafer bonding. After bonding and wafer splitting, the SiGe layer is removed by oxidation and HF etching, enabling the formation of very thin and uniform strained silicon-on-oxide surface.
Thus, there is a need in the art for versatile and efficient methods of fabricating semiconductor heterostructures, including alternative active-area materials disposed over a common insulator platform, that would address formation of interface defects in a variety of lattice-mismatched materials systems. There is also a need in the art for semiconductor devices utilizing a combination of integrated lattice-mismatched materials with reduced levels of substrate interface defects for improved functionality and performance.